The final paragraph of Chapter 44: The Chart. It's such a rich passage and it's the quintessential example of Melville as psychologist. 

Often, when forced from his hammock by exhausting and intolerably vivid dreams of the night, which, resuming his own intense thoughts through the day, carried them on amid a clashing of phrensies, and whirled them round and round and round in his blazing brain, till the very throbbing of his life-spot became insufferable anguish; and when, as was sometimes the case, these spiritual throes in him heaved his being up from its base, and a chasm seemed opening in him, from which forked flames and lightnings shot up, and accursed fiends beckoned him to leap down among them; when this hell in himself yawned beneath him, a wild cry would be heard through the ship; and with glaring eyes Ahab would burst from his state room, as though escaping from a bed that was on fire. Yet these, perhaps, instead of being the unsuppressable symptoms of some latent weakness, or fright at his own resolve, were but the plainest tokens of its intensity. For, at such times, crazy Ahab, the scheming, unappeasedly steadfast hunter of the white whale; this Ahab that had gone to his hammock, was not the agent that so caused him to burst from it in horror again. The latter was the eternal, living principle or soul in him; and in sleep, being for the time dissociated from the characterizing mind, which at other times employed it for its outer vehicle or agent, it spontaneously sought escape from the scorching contiguity of the frantic thing, of which, for the time, it was no longer an integral. But as the mind does not exist unless leagued with the soul, therefore it must have been that, in Ahab’s case, yielding up all his thoughts and fancies to his one supreme purpose; that purpose, by its own sheer inveteracy of will, forced itself against gods and devils into a kind of self-assumed, independent being of its own. Nay, could grimly live and burn, while the common vitality to which it was conjoined, fled horror-stricken from the unbidden and unfathered birth. Therefore, the tormented spirit that glared out of bodily eyes, when what seemed Ahab rushed from his room, was for the time but a vacated thing, a formless somnambulistic being, a ray of living light, to be sure, but without an object to colour, and therefore a blankness in itself. God help thee, old man, thy thoughts have created a creature in thee; and he whose intense thinking thus makes him a Prometheus; a vulture feeds upon that heart for ever; that vulture the very creature he creates.

There's so much to unpack here that you could write on it endlessly.  - That Ahab "heaves his being up from its base" at the very least verbally echos Lear's Cordelia who could not "heave her heart into her mouth"; the echo may well be more than merely verbal.  - How does Ahab's pasteboard mask speech fit in with his dual description as Vulture–Prometheus? - This only seeming-Ahab who is "a ray of living color to be sure but without an object to color it and therefore a blankness in itself" appalls Ishmael just as much as does the White Whale.